In traditional mobile IEEE 802.16 networks, a given coverage area is typically divided into smaller areas referred to as cells. A base station (BS), often located at the center of the cell, provides coverage to the mobile stations (MSs) within the cell. The BS is connected to the core network via a backhaul connection, typically provided by a wired or point-to-point microwave link. Within each cell, the BS communicates with MSs that are associated with it, using a specific type of Point-to-MultiPoint (PMP) link in which a central device (e.g. the BS) is connected to multiple peripheral devices (e.g. MSs). This type of connection means that any transmission of communications such as data that originate from the BS is received by one or more MSs, while any transmission of communications that originate from any of the MSs is received only by the BS. Each BS manages the allocation of resources to enable it to communicate with the MSs served thereby, and informs the MSs about the resource allocations.
Unfortunately, the traditional deployment as described hereinabove suffers from certain inherent practical problems such as coverage discontinuity that exist due to shadowing and Non-Line-Of-Sight (NLOS) connections, low Signal-to-Noise-Ratio (SNR) at the cell edge, and non-uniform distribution of traffic e.g., in areas having sub-areas that are densely populated areas (hotspots). In addition, another challenge that is becoming widespread in recent years is the increasing demand for higher data rates required by the MSs which causes more and more BSs to serve smaller and smaller cells (i.e. the cell coverage area literally shrinks). Such a solution becomes costly since the provisioning of backhauling links to a large number of BSs is expensive. In order to meet the growing demand and the strict requirements for coverage, throughput and capacity increase, as well as to meet a reasonable cost structure, the deployment of Relay Stations (RSs) has been considered as a promising solution to IEEE 802.16 PMP networks as well as for other wireless networks. However, in order to implement this promising solution, it has been decided by the IEEE 802.16 Standardization body that the IEEE 802.16e Standard cannot provide an adequate solution and therefore it was required to extend the IEEE 802.16 Standard by drafting an additional Standard amendment that supports such type of operation with RSs, namely, the IEEE 802.16j Standard.
In order to better appreciate the IEEE 802.16j solution, one must first consider the nature of the Relay Station. The RS is an entity that is wirelessly connected to the BS on one side and to a group of MSs on the other. The connection to the BS is commonly referred to as the Feeding Link or sometimes as the Relay backhaul link, while the connection to the MSs, where the RS essentially functions as a BS for these MSs, is called the Relay Access Link (as it provides Access service to the MSs connected to it). The RS operates using a “store and forward” paradigm. It receives the data selectively at specific time/frequency allocations as determined by the BS, decodes and processes the data, and subsequently transmits (relays) this data using different air interface resources. The two major advantages of using the relaying concept are the increase in frequency reuse resulting from the fact that the BS and RSs within a cell may each communicate at the same time with different MSs while using the same frequency resources, and the reduction in the MSs (as well as BS) transmit powers due to the reduction in respective transmission ranges.
In order to better appreciate the IEEE 802.16j solution, one must also consider that operating an RS in a network that operates using Time Division Duplexing (“TDD”) paradigm poses constraints and challenges on the RS. IEEE 802.16e compliant systems operate by exchanging communication frames and the most common frame length is 5 ms duration (though other frame lengths are supported as well). Presently, the more widespread mode of operation of IEEE 802.16 compliant systems is Time Division Duplexing (TDD). TDD operation implies that the communication from the BS to the MSs (Downlink) and the communication from the MSs to the BS (Uplink) are carried out on the same frequency channel through the partitioning of each frame to a Downlink part and an Uplink part. As the difference between the transmit power and receive threshold of a RS can be in the order of 130 dB, TDD operation of an RS poses a problem since effectively the RS cannot transmit along its RS Access Link while receiving in its RS Feeding Link, or conversely the RS cannot transmit in its RS Feeding Link while receiving in its RS Access Link. This problem arises since it is not practical to consider isolation between the transmitting antenna and receiving antenna that would exceed 90 dB.
Still, this type of a solution of employing RS in wireless networks has gained some further popularity in the recent years with the introduction of broadband systems, having physical limitation of the cell size. Classical relays operate at the radio level, by amplifying the received signal and re-transmitting it, typically at a different frequency. Newer relays, sometimes referred to as Layer 2 relays, decode the signal and re-transmit it at a different point in time.
U.S. Pat. No. 5,883,884 describes a wireless communication system in which a base unit transmits outgoing TDM signals within a base transmission coverage area at a first frequency. Repeaters in the base coverage area receive the outgoing signal and retransmit it within respective repeater coverage areas at respective frequencies, maintaining the same time slot orientation in TDM format, where several levels of repeaters form a hierarchy covering the expanded range. The remote subscriber units located in a coverage area receive the strongest outgoing frequency signal from a repeater/base unit in a time slot assigned to that unit for a particular call. Incoming TDMA signals from remote units use the same time slots used in received outgoing signals. Each repeater receives outgoing signals from a lower level repeater (or from the base unit) at the transmission frequency of the lower level repeater, and immediately retransmits the signal in its own coverage at a different frequency. Incoming signals transmitted to any particular repeater from a remote unit in its coverage area, or from a higher level repeater, are at the outgoing transmission frequency for that repeater. The solution provided by this publication to reduce interruption during communications is that the repeaters and remote units switch between repeaters to communicate with the base unit depending upon received signal strength.
U.S. Pat. No. 7,386,036 discloses a wireless multi-hop system in which radio links between relays and users are optimized separately from the links between relays and base stations and in which multiple simultaneous data streams between relays and base stations are created. The system includes a base station (BS) connected to the core network with a link of wire line quality, relay stations (RS) connected to the BS with a first radio interface, and to subscriber stations (SS), with a second radio interface. The first and second radio interfaces can operate, at least in part, using the same frequency bandwidth, and the SS can also connect directly to the BS using the second radio interface if the BS is closer than any RS.
In our co-pending application published under US 20090252203, a method is provided for conveying wireless communications in a radio network using OFDMA or multi-carrier technologies. The relay station is capable of simultaneously transmitting (or receiving) communications to at least two recipients along a shared frequency channel.
Although deploying additional BSs may provide even better access link capacity than RSs, still, deployment of BSs with dedicated wired backhauls (or dedicated point-to-point microwave, millimeter-wave or optical backhauls) would not be the best cost-effective solution. On the other hand, when an RS is deployed, instead of a BS provided with a wired backhaul connection, no direct backhaul costs are involved. When the option is installing a BS with wireless backhaul, the deployment of an RS saves the need to purchase and maintain the microwave link equipment (as well as the purchasing of additional spectrum resources for the operation of that BS). Thus, the advantages of the less complex and lower cost RSs motivated IEEE working groups to develop the IEEE 802.16j Mobile Multi-hop Relay (MMR) Standard for increasing the coverage area and throughput of the IEEE 802.16e standard via the deployment of fixed or nomadic RSs.
Still, some major problems associated with the IEEE 802.16j are yet to be solved, namely, the need to purchase/modify equipment that is in compliance with the different communications patterns (frames) as defined by the IEEE 802.16j both for the BS and the RS.
In their article “An evolved Cellular System Architecture Incorporating Relay Stations”, published in IEEE Communications Magazine, June 2009 (pp. 115-121), J. Sydir and R. Taori address the problem of incorporating relay stations in such wireless networks. Still, by their solution the operators must modify their equipment (or even will have to develop and install new equipment) in order for their equipment to support the IEEE 802.16j Standard according to which their network could operate in a relay mode.
With all the advantages of using the IEEE 802.16j Standard for the Multi-hop Relay, it still has few other drawbacks. For example the new requirements make the RS as a new entity that is not comprised by existing building blocks of IEEE 802.16e Standard (e.g. the BS or MS) that is far more complex than it was for use according to the IEEE 802.16e Standard, rendering the implementation of a network that supports the IEEE 802.16j Standard to be more expensive, so that service providers often refrain from upgrading their IEEE 802.16e deployment to support IEEE 802.16j Standard due to the high costs involved. Due to the above, to date, no single equipment vendor has yet developed an RS conforming to the IEEE 802.16j Standard. The present invention seeks to overcome the above described problems.